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Servo Timer II
Introduction
Features
Acknowledgements
BUY NOW
Triggering
Configuration
Trigger Condition
Servo Start
Servo End
Normal Operation
Hardware
Circuit Diagram
Component List
Component Overlay
Connections
Standard
Trigger Options
External Trigger
Cascading Timers
Servo Motors
Power Supply
Power Consumption
Notes
Hints and Tips
Disclaimer

Introduction

The Servo Timer II was primarily designed for parachute deployment on water rockets, but can be used for other applications. The timer controls a single RC servo motor that can open a latch on a parachute door. Once triggered the timer waits a set amount of time before moving the servo motor. The timer has a number of configurable parameters along with several trigger options. It may also be used in other applications such as staging rockets, controlling camera direction etc.

Servo Timer II

Features

  • 6 - 9V operation
  • Single RC servo output - Can drive most common RC servo motors.
  • Configurable servo start and end positions
  • 0-12 second delay selectable in 1 second increments
  • Built in 2G acceleration switch for launch detect
  • External make or break wire trigger
  • External 3V - 12V hi/low trigger
  • External timer output
  • High-brightness 'ARMED' status LED visible in broad daylight from 50 feet.
  • Timers can be cascaded
  • ICSP connector for firmware updates
  • Weight: 6 grams
  • Dimensions: 25mm x 43mm (1 x 1.7 inches) - Fits in T8 FTC.

Acknowledgements

A big thank you goes to PK from Whooshtronics for his great support on this project. Also thank you to everyone who helped with ideas and suggestions on Australian Rocketry's forum. The Servo Timer II development has been covered on these forum threads: http://ausrocketry.com/forum/viewtopic.php?f=60&t=2840 and http://ausrocketry.com/forum/viewtopic.php?f=60&t=2799 and also on the Yahoo water rocket forum here: http://groups.yahoo.com/group/water-rockets/message/10843

Also a thank you goes to the water rocket community who have supported us over the years and given us feedback on the previous versions of this timer. We have taken your suggestions and incorporated them into this design.


Buy It Now - SOLD OUT

Updated: 26th May 2013 - Sorry but there are no more Servo Timer IIs left for sale, as they have all sold out. Thank you to everyone who has purchased the timer. We will continue to provide support for the timers. If you have any questions or comments please contact us.

If you would like to build your own, the STII PCB layout (in Eagle .brd format) and .HEX files are now available for FREE on request. All the SMD components can be substituted with through-hole components to make it easier to build on a prototyping board. (Sorry we don't have a through-hole PCB layout)

The section at the end of this article covers hints and tips on how to use this timer.


Triggering the timer

The timer can be triggered in a variety of ways depending on the application.

  • Built in G-switch: The built in G-switch will detect a launch when the timer experiences acceleration of over 2G. Depending on the application, such as when triggering occurs at burnout or apogee, or where the board cannot be oriented vertically, the built in G-switch trigger option can be disabled.
  • Break wire: A pair of contacts is available on the input connector to connect a loop of wire. Depending on the configuration, either a break in the loop will trigger the timer, or connecting the loop will trigger it. This allows the timer to be triggered by simple mechanical contacts or switches whose contacts are normally open or closed. External higher acceleration G-switches can also be directly connected to this input. Pressure switches used for detecting burnout may also be connected.
  • External 3-12V input signal: This input is routed through an opto-coupler isolating the timer from the triggering circuitry. This allows other circuits and flight computers operating on different power supplies to trigger the timer directly without additional circuitry. This input can usually be connected directly to a pyro output of a flight computer or altimeter.

The timer can be started by a variety of events depending on the application. Some trigger event examples include:

  1. Launch detect – This is typically achieved using a G or acceleration switch. Triggering occurs as soon as the rocket leaves the launch pad. Another variation to this is a set of contacts that are closed and an insulator is removed from between them during launch. The insulator is usually attached to the launcher.
     
  2. Burnout detect - This technique can use a pressure switch to detect when the pressure inside the rocket has reached atmospheric pressure, or some preset value. This is useful for staging rockets. This allows the rocket to deploy the second stage at the correct time regardless of how much pressure or water was used in the booster.
     
  3. Negative G detect – This is another technique used to detect when the rocket has stopped producing thrust and the rocket starts to slow down. An inverted mercury switch can be used here where the mercury floats upwards to make contact as drag continues to slow the rocket down after burnout. Negative Gs happen shortly after burnout.

We’ll leave it up to the rocket builder to come up with their own ways to trigger the timer.

Configuration

Before using the timer in an application for the first time it needs to have the trigger condition and servo motor start and end positions configured.

Configuring the Trigger condition

To change the timer's trigger condition, do the following:

  1. With the power OFF set the rotary switch to the "D" position.
  2. Turn the timer ON.
  3. The LED should flash 3 times.
  4. Now rotate the switch to one of the following settings:
     
    Setting Description
    0 Internal G-switch
    1 External trigger on Make wire
    2 External trigger on Break wire
    3 Opto-coupler input trigger on High
    4 Opto-coupler input trigger on Low
    5 Internal G-switch with manual ARM

    NOTE: See the Connections section for wiring details of each of the trigger conditions,

  5. Turn the timer OFF.

The value is automatically saved in the internal EEPROM. The next time the timer is turned on it will be configured with the new trigger condition.

Configuring Servo Positions

In normal operation when the timer is first turned ON the servo motor moves to the start position. When the timer expires, the servo motor moves to the end position.

Due to the large variety of RC servo motors available, and because there are numerous situations in which they can be used it is possible to set the start and end position of the servo. Up to 16 positions are possible over the range of movement of the servo motor. The timer produces timing from ~0.7ms to 2.4ms which should cover most RC servo motors.

A servo that has 180 degrees of travel can be positioned in ~11 degree increments, while a servo with 90 degrees of travel can be positioned in ~5 degree increments.

Configuring Servo Start Position

  1. With the power OFF, turn the switch to the "F" position.
  2. Turn the timer ON
  3. The LED will flash once.
  4. The servo motor will be positioned to the currently set start position. If you want to retain this position, turn the timer OFF.
  5. If you wish to change the position, turn the switch slowly to one of the 16 settings until the motor reaches the desired position. The setting will be automatically stored in the EEPROM.
  6. Switch the timer OFF. The new setting will be ready when the timer is turned back on.

NOTE: You should turn the switch to one of the 0-C positions after setting the motor position so that you don't accidentally enter the configuration change when the timer is powered on next time.

Configuring Servo End Position

  1. With the power OFF, turn the switch to the "E" position.
  2. Turn the timer ON
  3. The LED will flash 2 times.
  4. The servo motor will be positioned to the currently set end position. If you want to retain this position, turn the timer OFF.
  5. If you wish to change the position, turn the switch slowly to one of the 16 settings until the motor reaches the desired position. The setting will be automatically stored in the EEPROM.
  6. Switch the timer OFF. The new setting will be ready when the timer is turned back on.

NOTE: You should turn the switch to one of the 0-C positions after setting the motor position so that you don't accidentally enter the configuration change when the timer is powered on next time.

Normal Operation

  1. With the power OFF, set the timer to the desired time delay on the rotary switch: 0-C (0-12seconds). Note that when the switch is set in the D-F range when powering on the timer will enter configuration mode.

    Setting the timer to 0 delay, allows the timer to be used with other apogee detecting sensors such as the uMAD or a barometric altimeter, so that the servo is activated as soon as apogee is detected.

    Zero delay is also useful when the timer is used with burn-out detection sensors for activating a staging mechanism in multi-stage rockets.
  2. Turn the power ON. The servo motor will move to the start position.

    NOTE: You can then turn the timer off again at this point. This is useful for getting the motor set in the correct position when setting up the deployment mechanism in the stowed position.
  3. After 5 seconds the timer enters the ARMED state and the LED will light. In this state the timer starts monitoring the configured trigger condition. If you have chosen trigger option 5 with manual arm, you need to first close the contacts to arm the timer.
  4. When the trigger condition is met, the timer starts the delay.
  5. At the end of the delay, the servo motor is moved to the END position activating the deployment mechanism. The timer also outputs a high signal on the external output pin 4 on the JP2 connector and brings it low again at the end of the motor movement. You can use this output to drive external circuits.
  6. The timer then briefly flashes the LED every few seconds to remind you to turn the power off. We killed a couple of batteries during development when we forgot to turn off the power.

Video describing the operation of Servo Timer II.


Hardware

Circuit Diagram

The circuit diagram for the servo timer II is shown in Figure 1. Central to the design is the PIC16F628A microcontroller. The on-board oscillator is used to reduce the external component count.

Figure 1. - Circuit Diagram

Component List

Component

Description

Quantity

Digikey Part #

IC1

PIC16F628A Microcontroller

1

PIC16F628A-I/SO-ND

SW2

HEX Rotary switch with real code.

1

563-1215-1-ND

IC2

LP38692MP-5 voltage regulator

1

LP38692MP-5.0CT-ND

IC3

PS2801 Opto-coupler

1

PS2801-1ACT-ND

R1

330 Ohm 0.125 watt resistor

1

P330ACT-ND

R2

10K Ohm 0.125 watt resistor

1

P10KACT-ND

R3

560 Ohm 0.125 watt resistor

1

P560ACT-ND

C1, C2

1.0uF Low ESR ceramic capacitor

2

445-1358-1-ND

SW1

Power switch (slide)

1

401-1999-1-ND

LED1

1000mCd red LED

1

754-1159-1-ND

SW3

G-switch Assemtech ASL2

1

RS Electronics 455-3665

JP1

6 pin right angle header

1

SAM1043-50-ND

JP2

4 pin right angle header

1

SAM1043-50-ND

J3

Battery clip

1

BS6I-ND

B1

PCB Board (ST_2_0)

1

-

Table 1. - Component List

Component Overlay

Figure 2. - Top Component Overlay

Figure 3. - Bottom Component Overlay

Standard Connections

The timer only needs a battery and an RC servo motor for the standard configuration. The timer comes standard with a 9V battery clip, but you can connect your own batteries to the timer.


Figure 4. - Standard connection

Trigger Options

The following diagrams show the different wiring connections needed for each type of trigger option. See the Configure trigger section for configuring the trigger option.

Trigger Option 0 - Internal G-switch

No external wiring is necessary for this option. The internal G-switch (shown in red) will trigger the timer when it undergoes acceleration of greater than 2G. A false trigger filter is implemented by the timer so small shocks should not cause the timer to activate. If you experience false triggers with your rocket configuration consider using some of the other trigger options such as trigger option 5..

IMPORTANT NOTES:

  1. The timer must be oriented vertically for the G-switch to trigger.

  2. If you are launching a rocket with a small nozzle and at low pressure the acceleration may be too low to trigger the timer using the internal G-switch. In this case we suggest using trigger option 1 or 2 to trigger the timer on launch.


Figure 5. - Trigger Option 0

Trigger Option 1 - External trigger on Make wire

Connecting pins 1 and 2 on the JP1 connector will cause the timer to trigger. You can connect any mechanical type contacts to these pins.


Figure 6. Trigger Option 1

Trigger Option 2 - External trigger on Break wire

Disconnecting pins 1 and 2 on the JP1 connector will cause the timer to trigger.


Figure 7. Trigger Option 2

Trigger Option 3 - Opto-coupler input trigger on High

A positive going pulse on pin 5 of the JP2 connector will cause the timer to trigger. You can connect the output of a flight computer, altimeter, uMAD or other powered sensors directly to this input. These include the pyro outputs from most rocket electronics.


Figure 8. - Trigger Option 3

Trigger Option 4 - Opto-coupler input trigger on Low

A negative going pulse on pin 5 of the JP2 connector will cause the timer to trigger.


Figure 9. - Trigger Option 4

Trigger Option 5 - Internal G-switch trigger with manual ARM

This trigger option allows manual arming of the timer. The internal G-switch is used to detect launch, but the timer is only armed once pins 1 and 2 of JP1 are connected. Normally the timer self arms 5 seconds after power-on. This allows the timer to be armed remotely if the process of pressurising the rocket is causing false triggers, or you need an extra secure trigger.

IMPORTANT NOTES:

  1. The timer must be oriented vertically for the G-switch to trigger.

  2. If you are launching a rocket with a small nozzle and at low pressure the acceleration may be too low to trigger the timer using the internal G-switch. In this case we suggest using trigger option 1 or 2 to trigger the timer on launch.


Figure 10. - Trigger Option 5

External Trigger

The servo timer can be directly triggered by other flight computers, altimeters or apogee detectors. The easiest way to connect the timer is to use the pyro ejection charge output on those devices. This trigger input is opto-coupler isolated so you can run the flight computers from a higher or lower voltage than the timer. The input is TTL compatible. 


Figure 11. Triggering from external devices

Cascading Timers

If you need to control two separate servo motors to do two different functions you can cascade the timers so that one triggers the other. You can connect both timers to the same battery, but keep in mind that if you are driving both servo motors simultaneously, the battery needs to have the capacity to do so.

Connect the Timer output (pin 4 on JP2) from the first timer to the External Trigger input+ (pin 5 JP2) of the second timer. Connect the ground wire (pin 3 on JP2) from the first timer to the External Trigger input- (pin 6 on JP2) on the second timer.

Now configure both timers as described above.

The external output signal of the first timer activates at the same time that the servo motor is activated. If you set a 0 delay on the second timer then both servo motors will move simultaneously.

You can cascade as many timers as will fit in your rocket.


Figure 12. Cascading Timers

Servo Motors

Standard and micro RC servomotors can be used with the servo timer and are available from most hobby stores. E-bay also has sets of servo motors at reasonable prices. The typical weight of the micro servos is anywhere from 3g to 15g, although larger ones can be used but you must ensure that the battery can deliver the higher currents. The timer supports currents of up to 1A for brief periods.

Power Supply

The servo timer can be powered by any DC voltage source in the 6V to 9V range. This includes 6V batteries, which can be made up of something like 2xCR123A's or 4x AAAA etc. a 9V battery, or a 2S LiPo battery pack at 7.4V. etc.

Power Consumption

The servo timer when armed and awaiting launch consumes ~10mA + servo standby power of around 6mA. When driving small servo motors, the current is likely to reach around 100-300mA. With a large stalled servo motor it could reach ~1A. This power is only applied for about 1second. After servo activation the timer + servo draws about 7.5mA.

Notes

  1. IMPORTANT: When using the on-board G-switch, you need to ensure that the PCB is oriented vertically and the G-switch is pointing up. If the PCB is not oriented this way, the servo timer most likely will not trigger.
  2. If you are launching rockets with small nozzles and low pressures then the rocket's acceleration may be too low to trigger the timer using the internal G-switch. In that case you can use the other trigger options to start the timer.
  3. You need to factor in that the deployment time is not the same as the parachute open time. It may take one or two seconds before the parachute is fully open, so if you are aiming for opening your parachute right at apogee, you need to start deploying it a little earlier. How much time it takes to fully open the parachute depends on the design of the deployment system, the parachute type and the parachute packing technique.
  4. Estimates for time delay settings is easiest done with a water rocket simulator that can predict the trajectory of the rocket.
  5. Adjustments to the timing can be easily achieved on subsequent flights.

Example of Servo Timer II used for parachute deployment

Servo Timer II used in conjunction with a uMAD for parachute deployment


Hints and Tips

This section will cover some tips on how to use the timers in deployment mechanisms and other applications.

Tip 1: Mounting the timer

6th July 2011

You can use regular machine screws to mount the timer to your deployment mechanism, but over the years we have found that during a crash the screws can put quite a bit of stress on the PCB and break it. We now almost always use just single core wire and thread it through the mounting holes and twist it. This holds the timer securely but during a crash it has some give. The wire ties will also save you a little bit of weight. For extra security you can use double sided tape under the timer and then tie it down with the wire.

Timer attached with wire ties.

Tip 2: Look out for dodgy servos

10th July 2011

When buying inexpensive servos take a look to see how they are assembled. The case normally comes in three parts. The cheaper variety tend to be just pressed together with little tabs. Under high G-loads the servo can fall apart and fail as shown in the photo below:

Some servos can fall apart under high G-loads.

This is particularly an issue when they are mounted vertically in the deployment mechanism. Hard impacts can also cause them to come apart. If you rough up the sides and epoxy a couple of pieces of plastic on both sides and make sure that all three sections are joined together then you can use the servos without issues.

Better still find servos that have screws that hold the three sections together as shown below:

Better servos have the case screwed together.

Tip 3: Mounting a micro servo motor

10th July 2011

Depending on your deployment mechanism you will need to mount your servo motor somewhere. We used to use the servo to pull a pin on the release mechanism, but the main draw back was that you needed to have a servo powerful enough to overcome the friction of the pin in the latch. This was particularly an issue if the parachute ejection spring was putting a large force on the latched door. Since then we have switched to using the servo to simply let go of a rubber band holding the door. This allows you to use even the smallest sub micro servos to release a parachute door, as the pulling force is in the direction the servo is moving. In this arrangement the servo is mounted centered around the door.

We simply cut a hole in the Coreflute for the motor to just slide in and secure it with a small self tapping screw. These often come with the servo motors. You can use a couple of screws if you like.

NOTE: Leave a little tab on the servo horn to prevent the rubber band from slipping over the servo axel. Without this tab it is possible that the rubber band will get caught on the servo horn. The following sequence shows how the servo releases the door.

The rubber band holds the parachute door closed.

As the servo starts moving the little tab pushes the rubber band to one side.

The rubber band slides down the servo horn.

Until it completely slips off opening the door and deploying the parachute.

With the super tiny sub-micro servos you want to get the rubber band as close to the axel as possible while in the stowed position as the rubber band can exert a high enough torque on the servo horn to turn it by itself. You can also turn the servo horn so the start position is such that it points almost all the way around and then half the rubber band goes over the little tab and under the big arm. This spreads the torque either side of the axel and will not turn it. When the motor starts moving it will swing the rubber band over and release the parachute.

Tip 4: Using the timer to deploy a parachute

8th September 2011

Here is a full tutorial on how to use the timer to deploy a parachute on a water rocket.

Tip 5: Simple Break-wire launch detect trigger

25th November 2011

If you are going to attempt fairly slow launches such as restricted nozzles, or foam you can use the break-wire option on the timer. All you need is a simple connector and two wires. You can buy these from a store, but if you have an old computer or other piece of electronic equipment that you are about to throw out, look inside, as often the front LEDs, speaker, buttons etc. are often connected to the mother board with one of these.

 

Just snip it off and strip the ends - about 1 cm. You can tin them if you like but it's not necessary. You can just twist the strands together.

Make a knot in the wires about 2-3 cm from the stripped ends. This will stop the wires from pulling out during launch.

Program the timer for Trigger Option 2 and connect the connector to pins 1 and 2 on JP1.

Make a small hole in the fairing, just big enough for the two wires to fit and thread them through. The knot should prevent them from coming out any further.

Securely attach one end of a piece of string to the launcher, and tie a loop in the other end. Thread the loop of string onto one of the wires and then twist them together about 1.5 times. Turn on the timer and wait for it to arm. When the rocket launches, the loop will pull through and break the wire connection triggering the timer. The break only needs to happen momentarily to trigger the timer. It does not matter that the wires touch afterwards.

Make sure you test the setup a couple of times before your first launch.

NOTE: Don't twist the wires too many times otherwise you may rip the nosecone off the rocket. :)


Disclaimer

In no respect shall Air Command Water Rockets incur any liability for any damages, including, but limited to, direct, indirect, special, or consequential damages arising out of, resulting from, or any way connected to the use of the item, whether or not based upon warranty, contract, tort, or otherwise; whether or not injury was sustained by persons or property or otherwise; and whether or not loss was sustained from, or arose out of, the results of, the item, or any services that may be provided by Air Command Water Rockets.

While we try to ensure the quality of the Servo Timer II, we cannot guarantee a rocket's safe return to Earth since the timer is fitted in recovery systems beyond our control. This product should always be considered experimental.

28/6/2011

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